High burning rate nf propellants

ABSTRACT

HIGH BURNING RATE SOLID PROPELLANTS HAVING NF-CONTAIN- ING BINDERS. THE PROPELLANT BINDER IS PREFERABLY COMPOSED TOTWO NF2-CONTAINING MATERIALS, THE POLYMER OF (2,3BIS(DIFFLUORAMINO)PROPYL ACRYLATE) AND 1,2,3-TRIS(A,B-BIS(DIFLYORAMINO)ETHOXYL) PROPANE PLASTICIZER. THE PROPELLANT OXIDIZER IS PREFERABLY AMMONIUM PERCHLORATE IN FINELY DIVIDED FROM AND A ALUMINUM POWDER IS THE PREFERRED SOLID PROPELLANT FUEL.

United States Patent 3,813,305 HIGH BURNING RATE NF PROPELLANTS Mart G. Baldwin and James L. Chaille, Huntsville, Ala.,

assignors to the United States of America as represented by the Secretary of the Army No Drawing. Filed Oct. 31, 1967, Ser. No. 679,968 Int. Cl. C06b 11/00 US. Cl. 149-193 8 Claims ABSTRACT OF THE DISCLOSURE High burning rate solid propellants having NF-containing binders. The propellant binder is preferably composed of two NF containing materials, the polymer of [2,3- bis(difiuoramino)propyl acrylate] and l,2,3tris[a,fi-bis- (difluoramino)ethoxy] propane plasticizer. The propellant oxidizer is preferably ammonium perchlorate in finely divided form and aluminum powder is the preferred solid propellant fuel.

BACKGROUND OF THE INVENTION This invention relates to a novel high burning rate solid propellant and particularly to one with a high energy binder.

A large amount of experience exists in the area of high burning rate solid propellants. Most prior work has dealt with the effects of propellant additives on the burning rate. The additives have included both chemical burning rate catalysts and short metallic fibers (staples). The use of chemical modifiers has often led to poor stability and/ or reduced specific impulse. The use of metallic fibers has increased processing difliculties and led to poor ballistic firing traces. Accordingly, a high burning rate homogeneous propellant is greatly desirable.

High burning rate homogeneous propellants which are easily castable have a number of applications in military weapons systems. Such high burning rate propellants are particularly suitable for use in high thrust to mass ratio small diameter motors.

Accordingly, it is the principal object of this invention to provide a high burning rate propellant which maintains good stability, processing and ballistic properties.

It is a particular object of this invention to provide such a high burning rate propellant which is homogeneous and has a high energy binder.

SUMMARY OF THE INVENTION A high burning rate propellant may be formulated with an NF-containing binder. The propellant binder is preferably composed of two NF -containing materials, the polymer of [2,3-bis(difluoramino)propyl acrylate] and 1,2,3 tris[a, 9 bis(difluoramino)ethoxy] propane plasticizer. The oxidizer is preferably ammonium perchlorate in finely divided form (8 micron average particle diameter). Aluminum powder is the preferred supplementary solid fuel.

DESCRIPTION OF THE PREFERRED EMBODIMENTS High burning rate solid propellants may be prepared with high energy NF-containing binders. The propellant binder is preferably composed of two NF -containing materials, the polymer of [2,3-bis(difiuoramino)propyl acrylate] and 1,2,3 tris[a,B bis(difluoramino)ethoxy]propane plasticizer. The oxidizer is preferably ammonium perchlorate in finely divided form (8 micron average particle diameter). Aluminum powder is the preferred supple mentary solid fuel.

3,813,305 Patented May 28, 1974 ice 2,3-bis(difluoramino)propyl acrylate has the following structure:

H- l-H H H 1% 2,3-bis(di-fluoramino)propyl acrylate can be prepared according to a procedure such as the following:

To a 200 ml. three-necked flask fitted with mechanical stirrer, condenser, thermometer and dropping funnel (all outlets covered with Drierite drying tubes) is introduced 7.72 parts (0.029 mole) of tn'phenylphosphine and 100 ml. anhydrous diethyl ether (commercial grade). To this solution is added 11.0 parts (0.029 mole) of 2,3-bis(difluoramino)propyl 0a,}9-dibIOIDOP1'OPiOI18tB. A slightly exothermic heat of reaction (23 C. to 27 C.) and an immediate precipitate of triphenylphosphine dibromide are noted. The addition is completed within a ten-minute period. The reaction is continued for a period of twentyfour hours. The solids are removed by filtration and the ether at reduced pressure on a rotary evaporator. The residue, containing a small quantity of solids, is treated with 100 ml. of pentane and filtered again. The pentane is removed and the residue first examined by gas chromatography and then distilled. The gas chromatogram showed the product to be approximately pure. The product (B.P. 52 C., 1.1 mm. of mercury) is obtained in a yield of 4.22 parts (67%) and is identified as 2,3-bis- (difiuoramino)propyl acrylate.

This and other procedures for the preparation of 2,3- bis(difluoramino)propyl acrylate are disclosed in US. Patent Application Ser. No. 360,168, filed Apr. 13, 1964, entitled Process for the Preparation of Nitrogen and Fluorine-Containing Acrylates and Methacrylates.

1,2,3 tliS[oz,/3 bis(difiuoramino)ethoxy] propane can be prepared by the reaction between tetrafiuorohydrazine and trivinoxypropane according to a procedure such as the following:

To a glass Aerosol tube of ml. capacity is introduced 5.14 g. (0.0 3 mole) of trivinoxypropane in 30 ml. of CCl.;. The tube is then placed in position in a high pressure manifold, degassed thoroughly under vacuum, flushed and degassed three times with nitrogen. Tetrafluorohydrazine is introduced into the reactor tube to give an initial pressure of 84 p.s.i.g. An initial pressure drop is observed which is due to solvent absorption. After recharging to 84 p.s.i.g. with tetrafluorohydrazine, the mixture is heated to 58 C. During the next eight hours, the tetrafiuorohydrazine pressure is maintained between 84- 29 p.s.i.g. by recharging the system at frequent intervals. The number of required recharges totals fifteen. The heating bath is lowered and after cooling the reaction mixture the excess tetrafiuorohydrazine is vented to the air. The reactor is degassed and opened to the air. Finally, the Aerosol tube with its contents is removed from the manifold and poured into a 100 ml. flask. The solvent is removed on a rotary stripper at reduced pressure. The residue product weighs 13.7 g. (96% yield) and is identified as l,2,3-tris[a,[3-'bis(difluoramino) ethoxy]propane.

This and other procedures for the preparation of 1,2,3- tris-[a,B-bis(difiuoramino)ethoxy] propane are disclosed in US. Patent Application Ser. No. 363,680, filed Apr. 27, 1964, entitled A Method for the Preparation of a Derivative of Trivinoxypropane.

To prepare a solid propellant having a binder such as the above homopolymer, the 2,3-bis(difiuoramino)propyl acrylate monomer is mixed with the plasticizer and the other propellant ingredients in the presence of a polymerization catalyst such as benzoyl peroxide. Curing is then effected at 50 C. for a period of about 40 hours.

The use of an NF -containing plasticizer such as 1,2,3- tris[a,/3-bis(difluoramino)ethoxy] propane is optional. However, this yields maximum burning rates.

Polymers other than the polymer of [2,3-bis(diliuoramino)propyl acrylate] and a plasticizer may be employed. However, the monomer must be an NF -containing compound.

A propellant composition which has been found especially desirable contains 12.5% (by weight) of 2,3-bis- (difiuoramino)propyl acrylate, 37.5% 1,2,3-tris[ot,j8-blS- (difiuoramino)ethoxy] propane plasticizer, 10.0% aluminum, and 40.0% ammonium perchlorate. It is to be understood that this example is merely given for purposes of illustration and is not intended to be a limitation of this invention.

A 100 gram batch of the above propellant composition was mixed and cast into .75C .5-l.5 inch treaded micromotors*. The motors were fired at a static test range using different size nozzles to give a range of pressures. When the burning rates obtained were plotted on log-log paper against pressure, a straight line resulted with a slope of about 0.6 and burning rates at 1000 p.s.i. of 2.05 in./sec. and at 10,000 psi. of 10.0 in./sec. Other propellant compositions of this invention were tested and similar results were obtained Generally speaking, the oxidizer is present in amounts of about 30% to about 60% by weight in the propellant composition, the polymer in amounts of about 8% to about 16%, the metallic fuel in amounts of from to about 20%, and the plasticizer in amounts of 0% to about 40%. Preferably, the plasticizer is present in an amount of at least about 20%, preferably about 40%. The plasticizer should be used because of the physical properties it imparts to the propellant composition. The plasticizer is preferably an .NF -containing material such as 1,2,3-tris[a,fi-bis(difluoramino)ethoxy] propane or 1,2,2,5,6,9,9, l 0-octakis difluoramino) -4,7-dioxadecane for maximum burning rates. However, non-NF -containing plasticizers such as triethylene glycol dinitrate may also be employed together with an NF -containing polymer, with considerably lessened impulses. t

1,2,2,5,6,9,9,10-Octakis(difluoramino)-4,7-dioxadecane, also known as 1,2-di-[2,2,3-tris(difiuoramino)propoxy1- l,2-bis(difiuoramino)ethane, is represented by the following structure:

The complete process for the preparation of this compound involves the following series of reactions:

(B CMD 0) The ID. of the motor, without propellant, is 0.75 inches. This is the same as the OD. of the propellant. C indicates a cylindricaily-shaped charge. 0.5 inches is the core diameter of the motor. 1.5 inches is the length of the motor.

In reaction 1) 4,4"bis(chloromethyl)-2,2'-bi-1,3-dioxolane (BCMDO) is prepared from 3-chloro-l,2-propanediol and crystalline-+% glyoxal using Amberlyst-lS (ion exchange resin) acid catalyst. At first the BCMDO was prepared according to the literature using 30% aqueous glyoxal and sulfuric acid catalyst, but here some difficulty was encountered with hydroxyl and carbonylcontaining impurities. Consistant yields of about 95% are obtained in the current method and it is found unnecessary to distill the crude BCMDO. 4,4-Dimethylene- (2,2'-bi-1,3-dioxolane) (BMDO) is obtained in reaction (2) by reaction of the BCMDO with sodium methoxide in methanol and dimethyl sulfoxide. The yield of crude BM'D'O is about and the purity is such that further purification by distillation is not necessary.

4,4-Bis(difiuora-mino) 4,4 bis(difluoraminomethyl) (2,2'-bi-1,3-dioxolane) (BMDOA) in reaction (3) is prepared from BMDO (in chloroform) and tetrafluorohydrazine (N F in a high pressure-rocking bomb static reactor. The production of about 60 grams per run is a convenient scale, the yields averaging about of the theoretical. The production of BMDOA was at first attempted in a high-pressure liquid phase flow reactor, but considerable difficulty was encountered because of the tendency for a solid material to be deposited in the reactor lines causing a plug. The solids may arise from at least two sources: an insoluble stereoisomeric form of BMDO and degradation of the olefin by hydrogen fluoride.

The final step (reaction 4) consists of adding BMDOA in Freon-113 (1.44 ml. of solvent per gram of adduct) to a mixture of HNF and 30% fuming sulfuric acid. The HNF is generated immediately prior to use from aqueous difluorourea.

The reaction is followed by a washing step during which the product is isolated in a solution of methylene chloride. The crude product is further refined by column chromatography using silica gel and 6/1 pentane/ methylene chloride solvent. The chromatography is effective in removing an unknown NF-containing impurity. Whether or not this impurity has a detrimental effect in propellant formulations has not been determined.

A typical example of the HNF; reaction follows:

Reagents 250 g. (0.661 moles) of BMDOA dissolved in F113 total sol., 500 ml.).

1720 ml. of 2 molar difluorourea (DFU) (3.44 moles of HNF 500 ml. of 2 M H SO for generator.

372 ml. of 30% F.H S0 (2.644 moles of S0 Apparatus The apparatus, which is assembled in a bomb-proof shelter to be operated remotely, consists of the following:

(1) PINE-Generator. (2) HNF -'Reactor. (3) Washing assembly.

The HNF generator consists of a 3-liter, 3-necked flask equipped with a nitrogen inlet tube, DFU addition container (constructed of polyethylene), heating mantle, and chilled water condenser (8-10 C.) to which is connected an ice-salt trap and drying tower (filled with indicator Drierite) respectively. The drying tower is connected to the HNF -reactor.

The l-lNF -reactor consists of a 1000 ml. indented flask equipped with a Friedericks condenser through which is circulated methylene chloride (cooled to approximately 70 C. by means of a Dry-Ice acetone bath), a chilled water condenser connected to the 70 C. condenser (an exit tube is connected to the chilled water condenser), mechanical stirrer, 500 ml. dropping funnel, a solvent inlet, a well for a temperature-indicating probe (thermistor), and a water bath containing a heating coil and chilled-water inlet. The reaction flask also has an outlet tube extending from the bottom of the flask through which nitrogen can be used to purge the reactor. The tube is fitted with a Teflon stopcock. A polyethylene line leads from the tube to the washing assembly.

The washing assembly consists of a 2-liter indented flask equipped with a mechanical stirrer and inlets for water and aqueous sodium bicarbonate. A 2-liter separatory funnel is located just below the washing flask to facilitate the washing operation. Transfer of the reaction solution is made by means of vacuum (water aspirator). A glasscentered (coarse) funnel is located below the separatory funnel for drying and filtering the washed solution.

Procedure After the reaction apparatus is checked and put in good working order, the 2 M sulfuric acid is placed in the generator along with a few boiling chips, water to the genera tor condenser (6-8 C.) is turned on, and purging with nitrogen of the generator and reactor is begun. The solvent container is filled with methylene chloride, the DFU is placed in the storage container, and the BMDOA solution is placed in the dropping funnel. When the nitrogen purge has proceeded for about ten minutes it is cut off and the 30% fuming sulfuric acid is added to the reactor. Circulation of methylene chloride (70 C.) to the condenser on the reactor is begun, the nitrogen fiow rate to the generator is adjusted to about 40 cc./minute and the nitrogen to the reactor is adjusted to a slow bubble (about cc./min.). About 600 ml. of DFU is added to the generator, heating of the generator is begun, the reactor chilled water-bath is brought to 68 C., and the reactor stirrer is begun. The generator temperature is maintained at approximately 100 C. (boiling) and the reactor bath temperature at 6- 8 C. during the addition of DFU. The remaining DFU is added in 150200 ml. portions in 10-15 minute inter vals. The total time taken for the DFU addition amounts to 1.25 hours. The exotherm produced from HNF combining with the fuming sulfuric acid raises the reactor temperature to about C. The maximum allowable exo' therm temperature is C.

When all of the DFU is added to generator and the reactor temperature has dropped to about 8l0 C. (bath temperature, 68 C.) the dropwise addition of the BMDOA solution is begun. The exothermic reaction is controlled by the addition rate of the BMDOA. The reactor is not allowed to exceed 20 C. All of the BMDOA is added over a 60 minute period (heating of the generator is stopped when about one-half of the BMDOA is added). About fifteen minutes after all of the BMDOA has been added, the reactor temperature is gradually raised to 20 C. by bringing the bath temperature to 20 C. These conditions are maintained for one and one-half hours and then the reaction is terminated. The total reaction time after the BMDOA is added amounts to two hours.

The reaction is terminated and the excess HNF is eliminated. Methylene chloride solvent is added to the reactor; circulation to the --70 C. condenser is out 01f, the chilled water condenser to the reactor is cut on, and the bath temperature is raised to 55-60 C. After the solvent in the reactor has refluxed (36-39 C.) for 5-l0 minutes the HNF elimination is assumed completed. The reaction mixture is brought to 2025 C. and transferred to the washing assembly using a water aspirator vacuum. The reactor and transfer line are rinsed several times with solvent. The reaction mixture is stirred well, allowed to stand until a distinct separation occurs and the acid layer (lower) is separated and discarded. The organic layer is washed with water saturated sodium bicarbonate, again with water, dried with magnesium sulfate, and filtered. The product solution (700 ml.) contains about 294 grams of the crude product which amounts to a yield based upon the fluorine content of the BMDOA. The product solution is crude OPE.

The crude product is further refined by column chromatography using 50 mesh silica gel (bed: 2 inches diameter x 12 inches high) and 6/1 pentane/methylene chloride. The original solvent is evaporated from the product by means of a water aspirator. The chromatographed material amounts to 248 g. or a 76.6% yield based on the fluorine content of the BMDOA.

Oxidizers other than ammonium perchlorate may be used. The oxidizer is preferably a solid inorganic oxidizing salt. However, ammonium perchlorate is desirable for maximum burning rates.

If desired, burning rate catalysts (or other additives) such as copper chromite may be incorporated in the propellant composition in very small amounts.

Another propellant composition of this invention involves the use of 2,3-bis(difluoramino) propyl acrylate monomer and a second monomer such as hydroxypropyl methacrylate crosslinked with a diisocyanate such as toluene 2,4-diisocyanate or acrylic acid crosslinked with a diepoxide crosslinking agent, such as 3,4-epoxycyclohexylmethyl 3,4 epoxycyclohexanecarboxylate to form a copolymer. This copolymer (heteropolymer) replaces the polymer of 2,3-bis(difluoramino)propyl acrylate in the propellant compositions already given for the homopolymer system. In general, the second monomer may be acrylic acid or any substituted acrylate.

To prepare a solid propellant having a binder such as one of the above heteropolymers, the comonomers are mixed and polymerized in solution with the aid of a polymerization catalyst such as benzoyl peroxide. The cure is effected at 50 C. for a period of about 40 hours. The copolymer (heteropolymer) has a molecular weight of 5,00010,000 and is then mixed and heated at 50 C. with the plasticizer, crosslinking agent and other propellant ingredients to form the solid propellant.

In general, a diisocyanate is used as the crosslinking agent if the second monomer is an acrylate, whereas a diepoxide is used as the crosslinking agent if the second monomer is acrylic acid.

In this heteropolymer system, the ratio of 2,3-bis(difluoramino)propyl acrylate to hydroxypropyl methacrylate is about 90:10 to about :5 (by weight). If acrylic acid is used as the second monomer, instead of hydroxypropyl methacrylate, the ratio of 2,3-bis(difluoramino) propyl acrylate to acrylic acid is about 94:6 to about 96:4.

Thus, by the use of an NF-containing binder, a high burning rate homogeneous solid propellant has been developed. Not only does the solid propellant possess a high burning rate (greater than 2 inches/second at 1000 p.s.i.), but it also possesses a constant pressure exponent (0.6 to 0.7) over the pressure range from 1000 to 10,000 p.s.i.a. This latter property is unique among ammonium perchlorate-containing propellants and is of considerable importance because it greatly extends'the range of obtainable burning rates and operating pressures for castable ammonium perchlorate-containing propellants.

Both improved performance and grain simplicity are achieved by substitution of high rate, high energy NF-containing propellants for conventional propellants in certain missile systems. The NF is present in the propellant binder rather than being employed as an additive.

Various other modifications and variations of this invention will become readily apparent to those skilled in the art in the light of the above teachings, which modifications and variations are within the spirit and scope of this invention.

We claim:

1. A propellant composition comprising a cured intimate mixture of from about 30% to about 60% by weight solid, inorganic, oxidizing salt and from about 8% to about 16% by weight of a polymer selected from the polymer of [2,3 bis(difiuoramino)propyl acrylate], the polymer formed by reacting 2,3-bis(difluoramino) propyl acrylate with hydroxypropyl methacrylate, and the polymer formed by reacting 2,3-bis(difluoroamino) propyl acrylate with acrylic acid.

2. The propellant composition of claim 1 additionally comprising a plasticizer selected from the plasticizers consisting of triethylene glycol dinitrate 1,2,3-tris[a,;3-bis(difiuoramino) ethoxy] propane, and l,2,2,5,6,9,9,l-octakis- (difluoramino)-4,7-dioxadecane.

3. The propellant composition of claim 2 wherein said plasticizer is present in an amount of from about 20% to about 40% by weight.

4. The propellant composition of claim 3 wherein said 8 plasticizer is 1,2,3-tris[cap-bis(difluoramino)ethoxy] propane, and said inorganic, oxidizing salt is ammonium perchlorate.

5. The propellant composition of claim 3 wherein said plasticizer is l,2,2,5,6,9,9,10 octakis(difluoramino)-4,7- dioxadecane and said inorganic, oxidizing salt is ammonium perchlorate.

6. The propellant composition of claim 3 additionally comprising aluminum powder up to about 20% by weight.

7. The propellant composition of claim 2 wherein said selected polymer is formed from 2,3-bis(difiuoramino) propyl acrylate and acrylic acid crosslinked with a diepoxide.

8. The propellant composition of claim 2 wherein said selected polymer is formed from 2,3-bis(difluoramino) propyl acrylate and hydroxypropyl methacrylate crosslinked with a diisocyanate.

References Cited UNITED STATES PATENTS 3,332,811 7/1967 Guthrie et al 149l9 X 3,332,812 7/1967 Guthrie 149l9 X 3,341,596 9/1967 Rhodes et a1 14919 X 3,346,430 10/1967 Engel 149-19 X 3,346,546 10/1967 Beach et a1 149-19 X STEPHEN I. LECHERT, 111., Primary Examiner U.S. Cl. X.R. 

